The present invention involves a resistance thermometer measuring system having provisions for indicating the occurrence of a malfunction.
For observing the temperature in electrical equipment, such as for example, rotary electrical motors, it is common to build in resistance thermometers. Such a thermometer can be provided, for example, as part of the stator and/or one of the bearings. The thermometers are utilized in conjunction with a registering instrument. The registering instrument can be suitably provided with an alarm device, which activates an alarm when the temperature of the resistance thermometer exceeds a preselected level.
In general, the sensing member in such a resistance thermometer is formed by a resistance wire of metal with a positive temperature coefficient, i.e., the resistance of the wire increases with increasing temperature. This involves the drawback, however, that if an interruption or break in the thermometer or in the connection leads occurs, a thermal relay will regard this as an infinitely high temperature thereby triggering the alarm of the registering instrument. Thus, the alarm is triggered even if the temperature conditions in the machine are within a normal range. If the thermal relay is connected for release of the machine upon occurrence of a rising temperature, an interruption, or malfunction, in the resistance thermometer will also cause an undesired release. It, therefore, is desirable to obtain an indication of such a malfunction and to block an unjustified alarm and/or release in response to the malfunction. On the other hand, thermal relays functioning upon falling temperature will normally provide no information at all about such a malfunction in the resistance thermometer. Consequently, the temperature of the machine may fall to any low degree without the thermal relay ever starting to operate.